I replaced the intake gasket on my 2001 altima 2.4 and now the car is shaking even more now
What went wrong?

I'm not familiar with your car, but it's very important to torque upper and lower intake manifold bolts in the proper sequence and torque amount (lb.-ft.). Surfaces need to be properly cleaned and new O-rings installed. The work should only be done, while the engine is "stone" cold.